The present invention relates to the field of instrumentation and control. More particularly, the invention relates to a high-efficiency device that draws power and transmits a signal over the same conductors.
Two-wire transmitters and controllers are well known in the field of instrumentation and control. Generally, a two-wire transmitter is a low-power device located proximate a substance, and used to measure one or more conditions of the substance (e.g., fluid level, temperature, pressure, flow). A two-wire controller is a low-powered device used for controlling such conditions (e.g., a remotely operated valve). The transmitter and controller uses the same conductors both to receive power from a power source and to transmit and/or receive signals to or from one or more indicating and/or control devices (e.g., display, meter, programmable controller, computer).
In order to accomplish these functions, two-wire transmitters and two-wire controllers traditionally incorporate certain components. Two-wire devices typically are coupled to an external power supply by a pair of conductors that form a loop between the device and the power supply. Two-wire devices are also coupled to a transducer device. In the case of the transmitter, the transducer monitors the conditions to be measured. The transducer provides a signal to the transmitter proportional to the condition of the substance to be measured. Acting as a variable current sink, the effective series resistance across the transmitter varies so as to produce a change in the current drawn by the transmitter representative of the condition being monitored. In the case of the controller, the transducer controls the state of the condition. The controller provides a signal to the transducer proportional to the desired state of the condition.
Current industry standards place certain constraints on the operation of two-wire devices. One such constraint is that the current in the two-wire loop must be between approximately 4 milliamperes and 20 milliamperes under normal operating procedures. Moreover, it is desirable that a 4-20 milliampere transmitter be capable of operating on slightly less than 4 milliamperes and also be able to draw slightly more than 20 milliamperes to facilitate calibration. For example, in the case of a transmitter using HART(trademark) protocol, a 1 milliampere peak-to-peak AC current must be superimposed on the operating current, requiring the transmitter to be capable of operating at instantaneous currents as low as 3.5 milliamperes.
A second constraint requires two-wire devices to be capable of operating from a standard power supply, usually 24 volts direct current (DC). These power supplies often have intrinsic safety barriers which may have an internal resistance of several hundred ohms. In addition, two-wire devices often must operate in circuit loops that may have wire resistance up to a few hundred ohms. For example, if an indicating device is used, the total loop resistance often reaches 600 ohms, thus reducing the terminal voltage at the two-wire device to 12 volts DC when the loop current is 20 milliamperes. As a result of this limited voltage supply, power available to the two-wire device is severely limited.
A third constraint is that two-wire devices typically contain electronic circuitry, which must operate from a reduced voltage (e.g., 3, 5, 10 volts) that cannot vary as the available voltage changes. As a result, the transmitter must employ circuitry to reduce the voltage available from the loop to the required voltage levels. Because the amount of power provided to the circuitry influences its capability, speed and accuracy, the regulation circuitry must function with as little power loss as possible.
To date, this regulation process has been performed by a linear regulating circuit, or by a linear regulating circuit in series with a non-linear regulating circuit. These linear regulating circuits unnecessarily reduce the power available to the circuitry by dissipating power equal to the product of the current used multiplied by the difference between the input voltage and the voltage required to operate the measuring circuit. For example, for a measuring circuit operating on 10 volts DC where the transmitter receives 21 volts DC, the power associated with the additional 11 volts would be dissipated in the form of heat.
Therefore, it is one object of the invention to provide a two-wire device in which the available power is not reduced as a consequence of the required power conversion.
Many two-wire devices store energy in order to permit high, intermittent peak energy use without requiring sudden increases in loop current. When power is first applied to the two-wire device, local energy storage devices can cause high loop current to flow, called inrush current. Large inrush currents can trigger thyristor-type intrinsic safety barriers, and can interfere with digital signaling systems.
Therefore, it is another object of the invention to provide internal energy storage without causing large inrush currents.
The present invention provides a process control device that does not reduce the available power during the required power regulation. The process control device comprises a measuring circuit and a power regulator circuit. The measuring circuit, which is coupled to the power regulator circuit, produces a control signal indicative of a measured value. The power regulator circuit is created such that it does not limit available power to the measuring circuit. The process control device also may comprise a power control circuit coupled to the measuring circuit. The power control circuit redirects a portion of the available power from the power regulator circuit in proportion to the control signal produced by the measuring circuit. The process control device also comprises two or more conductors that are in electrical communication with the power regulator circuit and the power control circuit. These conductors deliver the available power to the power regulator circuit and the power control circuit, as well as receiving a first electric signal from the power regulator circuit and a second electric signal from the power control circuit. The first and second electric signal may be electric currents, whose combined value falls in the range of 4-20 milliamperes. In addition, the available power may be provided by a direct-current power source.
The power regulator circuit may comprise a non-linear, power regulator, for example, a switching regulator. The power control circuit may comprise a voltage to current converter. The control signal provided by the measuring circuit may be an electric voltage, and the measured value may be provided to the measuring circuit by a sensor, for example a transducer. The power regulator circuit may also comprise a current limiting circuit for reducing current surges present when the process control device begins to operate.
According to an aspect of the invention, a method is provided for use in a process control system. The method comprises receiving power, regulating the power with a power regulator circuit to a first value to operate a measuring circuit, providing to a power control circuit a control signal produced by the measuring circuit, and converting the control signal to an electric signal to operate an indicator. Notably, the power regulator circuit does not limit the power to the measuring circuit.
According to an aspect of the invention, a process control system is provided. The process control system comprises a sensor adapted to determine a measured value, a process control device (as described above) in electrical communication with the sensor, and a power source coupled to the process control device by two or more conductors. The conductors deliver the available power from the power source to the process control device, and receive an electric signal from the process control device. The process control system further comprises an indicating device for describing the electric signal. The indicating device is coupled to the power source and the process control device.